Methods of minimising the effect of integrity loss in hollow fibre membrane modules

ABSTRACT

A method and apparatus for reducing the effect of integrity loss in a hollow fibre membrane module, said module including a plurality of hollow fibre membranes ( 5 ), at least one end of the fibre membranes ( 5 ) being supported in a pot ( 6 ), the method including the step of increasing flow resistance of the liquid through the lumen ( 8 ) of the fibre membrane ( 5 ) in the region of the pot ( 6 ).

RELATED APPLICATIONS

This application is a continuation, under 35 U.S.C. § 120, ofInternational Patent Application No. PCT/AU03/00755, filed on Jun. 17,2003 under the Patent Cooperation Treaty (PCT), which was published bythe International Bureau in English on Dec. 24, 2003, which designatesthe United States and claims the benefit of Australian ProvisionalPatent Application No. PS 3006, filed Jun. 18, 2002.

FIELD OF THE INVENTION

The present invention relates to membrane filtration systems and inparticular to system using a plurality of porous hollow fibre membraneswherein loss of membrane integrity can lead to degradation of filtrationperformance.

BACKGROUND OF THE INVENTION

Consider a typical hollow fibre membrane module as shown in FIG. 1. Themodule consists of plurality of hollow fibre membranes 5 potted at leastat one end into a pot 6 having a length L. In order to calculate theflow from individual fibres the TMP (Transmembrane pressure P₁-P₂) isconsidered as acting across a total module resistance R to give a flowQ:TMP/RαQ (at constant temperature)Now in this typical model we can break the resistance down into:R=R _(m) +R _(pot) 3 and Q _(i)αTMP/(R _(m) +R _(pot))where Q_(i) is the flow emerging from the top of the intact fibre, R_(m)is the module resistance and R_(pot) is the resistance across the pot.

We can assume R_(m) is constant—a sort of average—though it will varydown the length of the fibre.

Now taking the case where a fibre is broken at the top pot (a worst casefor filtrate bypass). In this case:R_(m)=0 and Q_(b)αTMP/R_(pot)where Q_(b) is the flow of filtrate emerging from the top of the brokenfibre.

The ratio of the flow down a broken fibre to the flow down an intactfibre is calculated as follows: $\begin{matrix}{= {{Q_{b}/Q_{i}} = {\left( {R_{m} + R_{pot}} \right)/R_{pot}}}} \\{= {1 + {R_{m}/R_{pot}}}}\end{matrix}$

In the normal case R_(m)>>R_(pot)−typically 20. Thus it can be seen abroken fibre allows a significant amount of feed to contaminate thefiltrate and thus degrade filtration performance. Additionally,increasing the internal diameter of the fibre makes the problemmassively worse as typically R_(pot)αL/d⁴, where d is the diameter ofthe lumen and L is the length of the pot.

Accordingly, it is desirable to reduce the flow of filtrate from abroken fibre. Take the case where we increase R_(pot) (for instance byincreasing L or reducing d). The limit of Q_(b)/Q_(i) tends to 1. Thisis a highly desirable result. But increasing the length of the pot isundesirable in other ways—it increases the length of the module and theexpense of the module and process. The other option is to reducing theinternal diameter of the fibre in the pot.

SUMMARY OF THE INVENTION

It is an object of the present invention to overcome or at leastameliorate the problems of the prior art associated with integrity lossin hollow fibre membrane filtration systems or at least provide a usefulalternative.

According to one aspect, the present invention provides a method ofreducing the effect of integrity loss in a hollow fibre membrane module,said module including a plurality of hollow fibre membranes, at leastone end of said fibre membranes being supported in a pot, the methodincluding the step of increasing flow resistance of the liquid throughthe lumen of the fibre membrane in the region of the pot.

Preferably, the step of increasing the flow resistance is produced byreducing the inner cross-sectional area of the fibre lumen in the regionof the pot. For preference, the step of increasing the flow resistanceis produced by placing a porous layer in the flow path of the fibrelumen in the region of the pot.

According to a second aspect, the present invention provides a hollowfibre membrane module including a plurality of hollow fibre membranessupported at least at one end in a pot and having flow restriction meansin the lumens of said fibre membranes in the region of said pot.

Preferably, the flow restriction means comprise means for reducing theinner cross-sectional area of the fibre lumen in the region of the pot.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention will now be described, by way ofexample only, with reference to the accompanying drawings in which:

FIG. 1 is a schematic sectional elevation of a typical hollow fibremembrane module with an intact and broken fibre;

FIG. 2 is a similar view to FIG. 1 with the addition of a porous layerto the pot surface;

FIGS. 3A to 3K show enlarged schematic cross-sectional elevations ofvarious embodiments of the invention; and

FIG. 4 shows the results of a test performed on two modules toillustrate the operation of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 2 of the drawings, one preferred embodiment of theinvention is illustrated. A sinter or porous layer 10 is placed on topof the pot 6 to provide a further series resistance R_(pot2) to the poti.e.R _(pot) =R _(pot1) +R _(pot2)

An appropriate sinter 10 may have openings of microns in dimension andonly be a few millimetres thick. This method may reduce the Q_(b)/Q_(i)by a factor of 10.

Such an arrangement provides added benefits when used for membranefilter systems in a bio-reactor. The high solids feed in bio reactorsleads to the sludge actually plugging the filter and self sealing thebroken fibre totally.

The may be extended to the general case by replacing the sinter with amembrane with the same pore size as the hollow fibre membrane andenabling achievement of this self plugging capability even with lowsolids feeds.

It will be apparent the extra resistance of the sinter or membrane 10will require an extra pressure to maintain the module filtrate flow,however, this is only an operating cost not a membrane process operatingefficiency as it is operating over the pot assembly, not across thecompressible dirt layer on the membrane.

Fouling of this membrane sinter can be reduced by a regular chemicalcleaning backwash with chlorine or other suitable cleaners.

The membrane/sinter 10 is desirably in intimate contact with the pot 6to prevent sideways flow of filtrate/feed bypass. This may also beachieved with a replaceable sinter/membrane element.

A highly asymmetric membrane 10 with the large pore side contacting thepot 6 (so in normal filtrate flow the filtrate flows in the direction ofreducing pore size) is desirable.

As shown in FIGS. 3B-3K a variety of methods may be used to increase thepot flow resistance.

Referring to FIG. 3A a normal pot 6 without modification is shown. FIG.3B shows an increased length pot 6 which, while increasing pot flowresistance, has other disadvantages.

FIG. 3C illustrates providing the fibre 5 with a non porous coating 7adjacent the interface 8 between the fibre 5 and the pot 6. This servesto increase pot flow resistance while also moving the fibre failurepoint away from the fibre-pot interface.

FIGS. 3D and 3E show a further method of reducing flow by reducing theinner diameter of the fibre lumen 8 using a layer of material 9 appliedto part or whole of the inner surface 11 of the fibre lumen 8 in theregion encompassed by the pot 6.

One method of providing such a layer 9 is to coat the inside of thelumen 8 near the end of the pot 6 with a thin layer of material thateffectively reduces the diameter of the fibre lumen 8 at this point.This can be achieved by drawing up a material such as epoxy into the endof the fibre lumen 8 and then allowing it to run out again before it hastime to set, leaving behind a thin coating 9 on the inner fibre lumenwall 12 that can then set over time.

The embodiment shown in FIG. 3F illustrates smearing the surface of thepot with a suitable grout material 13 to reduce the diameter of thefibre lumen 8 adjacent its opening 14 from the pot 6.

FIG. 3G shows the insertion of hollow annulus 15, for example, a hollowpin, into the end of the fibre lumen 8 in the region of the pot 6 toreduce the cross-sectional area of the lumen 8 in the region of the pot6.

FIG. 3H shows the use of a porous layer of material 10 across the lumenopening 14 as also shown in the embodiment of FIG. 2.

FIG. 31 shows an embodiment where a porous material is forced into thelumen opening 14 to form a plug 16. This can be achieved by smearing aporous grout across and into the fibre lumen opening 14. Again thisserves to reduce the flow resistance of the fibre lumen in the region ofthe pot 6.

FIG. 3J illustrates an embodiment of the invention where the fibre lumen8 is narrowed within the region of the pot 6 by causing the pottingmaterial to swell or constricting the end of the fibre.

FIG. 3K shows an embodiment where the fibre lumen end is narrowed priorto potting.

FIG. 4 shows the results of a test performed on two modules toillustrate the operation of the invention. Two modules A and B were usedin the test. For each module one hollow fibre membrane was potted. Theend of the fibre which was not in the pot, was sealed. A stainless steelmesh was glued on the top of one of the pots in a way that preventedsideways flow of feed bypass during filtration in a similar manner tothe embodiments shown in FIGS. 2 and 3H. The mesh had openings of 51microns and was 56 microns thick. The characteristics of both of themodules are shown in Table 1. TABLE 1 Characteristics of the modulesLength of the pot Length of the fibre Name L_(p) (mm) L_(f) (mm) Othercharacteristics Module A 56 202 none Module B 53 205 Mesh glued on thepot

Firstly, feed water was filtered through module A for 35 minutes. Duringthis filtration, the transmembrane pressure (TMP) was measured. Then thefibre of module A was cut as close to the pot as possible and module Afiltered the same feed water for a further 35 minutes. During thisfiltration, the transmembrane pressure (TMP) was measured. The same testwas repeated with the module B using the same feed water.

The graph shown in FIG. 4 compares the TMP of the modules A and B duringthe two filtrations before and after the fibre was cut. The first partof the graph shows that the two curves are very similar. In particular,it shows that TMP of both modules increased at the same rate. Fibres ofthe modules were fouled at a similar rate. The small difference in TMPbetween the two modules is due to the mesh on module B which adds asmall extra resistance to flow. The second part of the graph after thefibre of modules was cut shows that TMP of module A and B developed in ahighly different way. The TMP of module A remained low and level whereasthe TMP of module B increased sharply showing that the mesh was blockedby the feed contaminants.

This test clearly shows the efficiency of a mesh as far as reduction ofintegrity loss is concerned. Due to the addition of the mesh to themodule, the cut fibre quickly sealed itself, preventing the feed fromcontaminating the filtrate.

It will be apparent to those skilled in the art that a wide variety andnumber of techniques can be used to reduce the flow within the fibrelumen in the region of the pot and that such techniques fall within thescope of the invention described. It will also be appreciated thatfurther embodiments and exemplifications of the invention are possiblewithout departing from the spirit or scope of the invention described.

1. A method of reducing an effect of an integrity loss in a hollow fibremembrane module, the module comprising a plurality of hollow fibremembranes, each hollow fibre membrane comprising a lumen, wherein atleast one end of each of the hollow fibre membranes is supported in apot, the method comprising the step of: increasing a flow resistance ofa liquid through a lumen of a fibre membrane in a region of the pot,whereby an effect of an integrity loss is reduced.
 2. The methodaccording to claim 1, wherein the step of increasing a flow resistancecomprises reducing an inner cross-sectional area of the lumen in theregion of the pot.
 3. The method according to claim 1, wherein the stepof increasing the flow resistance comprises placing a porous layer in aflow path of the lumen in the region of the pot.
 4. The method accordingto claim 2, wherein reducing the inner cross-sectional area of the lumencomprises applying a coating to at least a portion of an internalsurface of the lumen in the region of the pot.
 5. The method accordingto claim 2, wherein reducing the inner cross-sectional area of the lumencomprises smearing a surface of the pot in the region of the lumen witha grouting material.
 6. The method according to claim 2, whereinreducing the inner cross-sectional area of the lumen comprises insertinga hollow annulus into the lumen in the region of the pot.
 7. The methodaccording to claim 2, wherein reducing the inner cross-sectional area ofthe lumen comprises constricting an end of the lumen in the region ofthe pot.
 8. The method according to claim 7, wherein constrictioncomprises swelling a material used to form the pot.
 9. The methodaccording to claim 2, wherein reducing the inner cross-sectional area ofthe lumen comprises plugging an end of the lumen with a porous materialin the region of the pot.
 10. A hollow fibre membrane module comprisinga plurality of hollow fibre membranes, each hollow fibre membranecomprising a lumen, wherein at least one end of each of the hollow fibremembranes is supported in a pot, the hollow fibre membrane modulefurther comprising a flow restriction means for increasing a flowresistance of a liquid through the lumen of the hollow fibre membrane ina region of the pot.
 11. The hollow fibre membrane module according toclaim 10, wherein the flow restriction means comprises means forreducing an inner cross-sectional area of the lumen in the region of thepot.
 12. The hollow fibre membrane module according to claim 10, whereinthe flow restriction means comprises a porous layer in a flow path ofthe lumen in the region of the pot.
 13. The hollow fibre membrane moduleaccording to claim 10, wherein the flow restriction means comprises acoating applied to at least a portion of an internal surface of thelumen in the region of the pot.
 14. The hollow fibre membrane moduleaccording to claim 10, wherein the flow restriction means comprises agrouting material smeared on a surface of the pot in the region of thelumen.
 15. The hollow fibre membrane module according to claim 10,wherein the flow restriction means comprises a hollow annulus insertedinto the lumen in the region of the pot.